The present invention relates to a copier, printer, facsimile apparatus or similar electrophotographic image forming apparatus and more particularly to an image forming apparatus including a revolver or rotary developing device rotatable relative to an apparatus body and a drive source for controlling the drive and stop of the revolver.
A color image forming apparatus including a revolver rotatable relative to its body is conventional. To increase the operation speed and durability of such an apparatus, some different conditions must be satisfied, as follows. First, a period of time necessary for the revolver to switch the color for development must be reduced in order to drive the revolver at high speed. Second, the rigidity of the revolver must be great enough to withstand sudden starts and stops of rotation and to insure an accurate gap for development. Third, a developing roller must be provided with a larger diameter in order to increase the developing speed. Fourth, the amount of developer to be stored in the revolver must be increased to extend the life or maintenance period of the revolver. However, a revolver satisfying all of the above conditions is bulky and heavy and has therefore a great moment of inertia. It follows that a drive motor with greater power is required to drive the revolver at high speed.
The problem with the image forming apparatus of the type described is that when the vibration of the drive motor is transferred to the apparatus body, it effects image writing accuracy and thereby brings about so-called jitter in images. In addition, the apparatus body outputs annoying high frequency noise. In light of this, it is a common practice with the apparatus to interpose a motor mount or vibration absorbing member formed of rubber between the apparatus body and the mounting surface of the drive motor.
However, when the above motor mount is applied to a drive unit assigned to a revolver operable at high speed, the revolver overruns at the time of a stop and shifts a developing position, resulting in the color omission of an image. It was experimentally found that the overrun was ascribable to the shaking of the drive motor varying the distance between the axis of a revolver gear and that of a motor gear. The variation in the above distance caused the teeth of the two gears to skip each other when the revolver was decelerated.
Assume that the revolver with a great moment of inertia and rotating at the top speed is caused to stop by sharp acceleration. Then, a heavy load acts on the tooth surfaces of the motor gear. As a result, the motor mount elastically bears the displacement of the motor tending to move, or shake, away from the revolver gear. During repeated image formation, the motor mount formed of rubber generates heat due to the absorption of the motor vibration and the resistance to the load of the shaking of the motor. This, coupled with heat transferred from the motor, raises the temperature of rubber constituting the motor mount. The Young's modulus of rubber decreases with an increase in temperature and aggravates the above shaking of the motor shaft for a given load acting on the motor gear.
When the revolver is heavy weight, considerable speed reduction must be implemented by gears for a given rotation speed of the motor. It follows that to accurately stop the rotation of the revolver, the number of gears constituting a driveline should preferably as small as possible in order to minimize the influence of the backlash of gears. For a great speed reduction ratio, a driven gear must be provided with a great number of teeth relative to the number of teeth of a drive gear, so that the module of the gear is limited.
For the above reason, the gear teeth have a relatively small height and limits a margin as to the variation of the distance between the axis ascribable to the shake of the motor shaft. As the temperature of the rubber constituting the motor mount rises, the shake of the motor shaft ascribable to the load acting during deceleration increases and results in overrun for the long run. This is likely to occur particularly when the motor shaft is made long for layout reasons, when the rubber of the motor mount is made thick in order to sufficiently intercept vibration, or when the distance between the motor mounting surface and the gear is great.
The rigidity of the motor mount may be increased if the motor mount is provided with rubber having greater hardness or smaller thickness. However, the upper limit of rubber hardness is limited by fluidity during molding. Moreover, increasing the rigidity of the motor mount causes the motor mount to lose its vibration absorbing function.
The above problems occur with various kinds of rotary units including the revolver.
Technologies relating to the present invention are disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 7-99593, 9-120209, 9-230658, and 10-338380.